Tweeters are a type of speaker that is designed to reproduce higher audio frequencies typically from as low as 1.5 kHz to 20 kHz or higher. As is known to those of skill in the art, the volume of air behind a tweeter diaphragm helps determine the frequency and Q factor at resonance, as the air acts as a spring against the diaphragm. For this reason, the volume of air behind the diaphragm of a traditional tweeter is sealed, to prevent the air from escaping during operation and deteriorating the sound quality of the speaker.
Although this arrangement has functioned well for many years, the small shape and size of tweeters have made it difficult for them to reproduce frequencies below around 3000 Hz at a high output level without excessive distortion or thermal overload. For example, a 25 mm diaphragm on a sealed tweeter would need to oscillate a distance of 0.24 mm in order to produce 100 dB SPL at 1 meter at 3000 Hz, but would need to increase this travel 4 fold to 0.96 mm at 1500 Hz. Most tweeters of this design use an underhung voice coil design in order to maximize efficiency, and will start to produce excessive distortion once exceeding around 0.2 mm travel, a travel distance that can be maintained by use of a tuned duct.
Due to this limitation, many commercial speaker systems employ a bass/midrange driver or a dedicated midrange driver to cover the frequencies up to 3000 Hz or higher. Unfortunately there are a lot of compromises with this approach such as cone breakup and reduced high frequency dispersion of the bass/midrange driver or a more complex and expensive crossover and box and the reduced efficiency of most dedicated midrange drivers.